P Advances in Understanding and Managing Dry Eye Disease ERSPECTIVE

PERSPECTIVE
Advances in Understanding and Managing
Dry Eye Disease
MICHAEL A. LEMP
● PURPOSE: To present evidence from the literature
and scientific meetings to support fundamental changes
in concepts regarding the prevalence, pathogenesis,
definition, diagnosis, management of dry eye disease
(DED) and the prospects for the development of new
therapies.
● DESIGN: Analysis and clinical perspective of the literature and recent presentations.
● METHODS: Review and interpretation of literature.
● RESULTS: The tear film and ocular surface form an
integrated physiologic unit linking the surface epithelia
and secretory glands via a neural network. This sensorydriven network regulates secretory activity in quantity
and composition, supporting the homeostasis of the
system. The tear film forms a metastable covering between blinks, subserving clear vision, and maintains the
health and turnover of the ocular surface cells. Disturbance of intrinsic factors such as increasing age; hormonal balance; systemic or local autoimmune disease, or
both; systemic drugs or extrinsic factors including topical
medications; environmental stress; contact lens wear; or
refractive surgery result in a final common pathway of
events at the tear film and ocular surface, resulting in
DED. Diagnosis of DED and the design of clinical trials
for new drugs have been hampered by a lack of correlation between signs and symptoms and flawed endpoints;
successful new drug applications likely will require new
approaches, such as the use of objective biomarkers for
disease severity.
● CONCLUSIONS: Recent advances in our knowledge of
the causation of DED open opportunities for improving
diagnosis and disease management and for developing
new, more effective therapies to manage this widely
prevalent and debilitating disease state. (Am J Ophthalmol 2008;146:350 –356. © 2008 by Elsevier Inc.
All rights reserved.)
Accepted for publication May 14, 2008.
From the Department of Ophthalmology, Georgetown University
School of Medicine, Washington, DC.
Inquiries to Michael A. Lemp, 4000 Cathedral Avenue NW, No.
828B, Washington, DC 20016; e-mail: [email protected]
350
©
2008 BY
STRUCTURE AND FUNCTION OF
THE TEAR FILM AND THE
OCULAR SURFACE
O
VER THE LAST TWO DECADES, SUBSTANTIAL
progress has been made in understanding the
structural elements of the tear film, ocular surface, and the associated tissues that form a single integrated unit termed the lacrimal functional unit.1 This
information has led to revised concepts about the way in
which the tear film is formed and maintained and the
pathophysiologic events operative in the development of
dry eye. In addition, it has opened paths for new therapeutic interventions.
Traditionally, the tear film has been thought to consist
of three discrete layers, with an innermost mucin layer
covering the corneal and conjunctival epithelium, an
intermediate aqueous layer produced by the lacrimal
glands, and an outermost lipid layer, the product of the
meibomian glands of the eyelids2; this concept has been
revised substantially. The contemporary concept of the
tear– ocular surface structure is that of a metastable tear
film consisting of an aqueous gel with a gradient of mucin
content decreasing from the ocular surface to the undersurface of the outermost lipid layer. The latter structure
interacts with the underlying aqueous and mucin components, retarding evaporative loss of aqueous tears and
contributing to the stability of the tear film between
blinks.3
The tear film is formed by a blink, which distributes the
tears over the ocular surface; immediately after the blink,
the tear film starts to thin in an orderly fashion, maintaining a complete aqueous cover until the next blink occurs,
reestablishing a thicker film, and the process repeats itself.
At least three distinct types of mucin have been identified:
transmembrane mucins produced by the corneal and conjunctival cells, gel-forming mucins from the conjunctival
goblet cells, and soluble mucins primarily from the lacrimal
glands.4 The transmembrane mucins contribute to the
surface structure of the epithelial cells, interact with the
gel-forming and soluble mucins of the tear film to stabilize
the film, and provide a cleansing pathway for the ocular
surface; lipid–mucin interactions support a relatively stable
tear film between blinks.
ELSEVIER INC. ALL
RIGHTS RESERVED.
0002-9394/08/$34.00
doi:10.1016/j.ajo.2008.05.016
In addition to nourishing the ocular surface and providing for lubrication between the lids and the ocular surface,
the tear film serves as the anterior refracting surface of the
eye. Recent studies have demonstrated the profound effects on vision when the tear film becomes unstable in dry
eye disease (DED; vide infra). All tissues of the ocular
surface, secretory glands, eyelids and outflow channels of
the nasolacrimal pathway are linked via a neural network
(the lacrimal functional unit).1 Sensory receptors monitor
conditions of the tears and cells, sending afferent signals to
the central nervous system that, in turn, send efferent
impulses to the secretory glands and cells, effecting
changes in composition and volume to maintain homeostasis and to respond to stress and injury. Additional
factors supporting the tear film– ocular surface complex
include bioavailable hormones, primarily androgens, and
an intact immune system. This exquisitely balanced system
represents a highly complex unit providing our visual
access to the external environment.5 Derangement of any
one element leads to a breakdown in overall structure and
function with significant clinical effects.
CHARACTERISTICS OF DRY
EYE DISEASE
THERE ARE A NUMBER OF RECOGNIZED RISK FACTORS FOR
the development of dry eye. These include: aging; female
gender; hormonal changes; systemic autoimmune disease
(most prominently Sjögren syndrome); decreased corneal
sensation; refractive surgery in which the corneal nerves
are either severed or ablated; blinking abnormalities; drug
effects; viral infections such as human immunodeficiency
virus, cytomegalovirus, and hepatitis C; diabetes mellitus;
vitamin A deficiency; and graft-versus-host disease.5 Regardless of which of the initiating factors or groups of
factors result in the presentation of dry eye, there is a
common final pathway for expression of the disease at the
tear film– ocular surface interface. Common features include: an unstable tear film between blinks, elevated
electrolyte concentration in tears leading to hyperosmolarity and subsequent damage to the ocular surface, symptoms of discomfort, and a decrease in vision between
blinks. Inflammation is a feature in dry eye associated in
both Sjögren-associated and non–Sjögren-associated
DED.6 It has been reported that allergy and other inflammatory conditions of the ocular surface can destabilize the
tear film.7 Although the exact place of inflammation in the
stream of events leading to ocular surface distress is not
clear, its role is unmistakable.
DRY EYE AS A DISEASE
DRY EYE HAS A NUMBER OF NAMES ASSOCIATED WITH IT.
These include: keratoconjunctivitis sicca, dry eye synVOL. 146, NO. 3
ADVANCES
IN
drome, and the more recently suggested dysfunctional tear
syndrome. Dry eye develops in response to the presence of
one or more risk factors listed above; in addition, environmental, workplace, or recreational stress—for example,
arid atmosphere, constant wind currents, the presence of a
contact lens, and prolonged use of video display screens—
are factors that can initiate and exacerbate the disease
process. The features of dry eye are those of a specific
disease process, and dry eye is, therefore, a disease. The use
of the term syndrome, which is a collection of presenting
signs usually applied to multiple organ systems, tends to
trivialize a discrete and debilitating disease.
The term dry eye has been criticized recently as not
being fully descriptive of a process that, in some patients,
may be characterized primarily by qualitative changes in
the tear film, and the substitute dysfunctional tear syndrome
(DTS) has been proposed.8 Although this term is arguably
more descriptive, dry eye is embedded not only in the
medical literature but also in lay writing and is used in
other languages. At a recent large international dry eye
workshop, DTS was rejected as a substitute,9 and the term
dry eye disease was accepted and is used in the recently
published Report of the International Dry Eye Workshop
(DEWS).8
PREVALENCE OF DRY EYE DISEASE
IT HAS BEEN KNOWN FOR MANY YEARS THAT DED IS A
common clinical problem. Only recently, however, have
valid quantitative data appeared that document the extent
of DED. Surveys over the last 20 years have estimated the
prevalence of DED to be between 5% to more than 30% at
various ages.5 Different definitions of the disease at use in
various studies make comparison difficult. In a survey by
the American Academy of Ophthalmology, respondents
reported that approximately 30% of patients seeking treatment at an ophthalmologist’s office have symptoms consistent with DED.10 In several large studies, it is estimated
that just fewer than 5 million Americans 50 years and
older have moderate to severe DED.11 Other estimates,
which include subjects reporting dry eye symptoms some of
the time or in response to certain environmental, workplace, or recreational activities, range as high as 20% of the
American population. It is thought that European and
Asian populations have a similar or slightly higher prevalence. With the aging of populations in developed countries, it is likely that the numbers of subjects with DED will
increase substantially. In younger subjects, the spread of
refractive surgery in which the corneal nerves are either
severed or ablated is associated with a high incidence of
postoperative DED.12 Although there is some debate as to
the extent to which this is true DED or a form of
neurotrophic keratopathy,13 symptoms of DED occur in
more than 50% of laser in situ keratomileusis patients. A
DRY EYE DISEASE
351
significant group has continuing symptoms for months to
even years after surgery.14
Although irritation has been the primary symptom
associated with DED, other limiting factors relating to
vision loss have added to the impact on the quality of life
of patients. This impact has been measured in a number of
ways. Specific questionnaires measure effects of DED on
activities of daily life such as reading, computer use,
driving, pain and irritation, and general health and wellbeing.15,16 These have demonstrated a significant degradation in the quality of life in those with DED. Utility scores,
another measure of impact on quality of life, have shown
that patients with DED rate the severity of impact on their
lives as similar to those patients with moderate angina.17
DED is increasingly recognized as one of the most commonly encountered diseases with a substantial effect on
peoples’ lives and sense of well-being that limits important
daily activities and leads to a significantly reduced quality
of life.
Although it is intuitive to think that interruption of the
barrier function of the corneal epithelium manifest by
corneal fluorescein staining would lead to a susceptibility
to infection, my clinical experience is that microbial
infections are quite rare in the absence of the comorbid
factors mentioned above or other conditions such as
exposure keratitis, graft-versus-host disease, or other systemic immunologic disorders.
The risk for microbial keratitis in DED without these
other factors would seem to be of a very low order in that
given the wide prevalence of the DED, there are a limited
number of cases of microbial keratitis seen, and these are
usually associated with the comorbid conditions mentioned above. Corneal staining itself, especially in the
peripheral inferior cornea, commonly is seen in non-DED
subjects and usually is a late sign in DED patients (vide
infra).22 Were staining an indicator for risk for microbial
keratitis in DED, there would be many more cases than
practitioners encounter in practice. It is probable that the
redundant defense mechanisms at play in defending the
external eye against infection are effective even in dry eye
patients, unless they are compromised by one or more of
the additional risk factors mentioned.
DRY EYE DISEASE AND MICROBIAL
INFECTION
THE EXTERNAL EYE HAS A NUMBER OF DEFENSE MECHA-
nisms that protect the ocular surface against microbial
infection.18 These include mechanical factors such as
tearing and blinking, which remove noxious agents from
contact with the ocular surface. In addition, immunity
plays an important role. The immune system operating at
the ocular surface is complex, involving both an immediate local innate system comprising cells and mechanisms
that defend the host from infection by other organisms.
Protective cells include Mast cells, neutrophils, macrophages, dendritic cells, basophils, and eosinophils. Access
to systemic cells may be facilitated by local neurogenic
inflammation.19 In addition, immunomodulating proteins,
for example, lactoferrin, lysozyme, toll-like receptors, complement, neuropeptides, and many other of the more than
500 proteins contained within aqueous tears, form an
adaptive immunity mediated by systemic responses (e.g.,
T cells). Although the relative roles of these two forms of
immunity in the protection of the eye from noxious
influences are, as yet, unclear, their effectiveness is clear.
It is commonly thought that patients with DED are
more susceptible to microbial keratitis than the general
population. This is, however, poorly documented in the
literature.5 Most of the reports concern cases of patients
with comorbid conditions, for example, systemic autoimmune disease, particularly rheumatoid arthritis, or other
factors such as surgery, trauma, or contact lens wear.
Ocular surface disease or keratopathy is mentioned as a
predisposing factor, but no further characterization of the
condition is provided.20,21 This has given rise to a misleading impression that even small amounts of surface disruption may predispose patients to microbial keratitis.
352
AMERICAN JOURNAL
DRY EYE DISEASE AND ITS EFFECT
ON VISION
PATIENTS WITH SYMPTOMS OF OCULAR IRRITATION SUG-
gestive of DED often also report more vague problems such
as sensitivity to light, a decrease in reading, night driving
difficulties, or ocular fatigue. Only in the past several years
has it been recognized that these symptoms can be attributed to the effects of DED on vision. It is a common
clinical experience that standard visual acuity (VA) testing with Snellen or Early Treatment Diabetic Retinopathy
Study (ETDRS) charts seldom reveals a significant drop in
vision in DED patients until they exhibit moderate to
severe staining of the central cornea. Early in the course of
development of DED, however, the tear film becomes
unstable between blinks.23 An initial compensatory response to this is rapid blinking to reestablish momentarily
a continuous tear film necessary for clear vision; this allows
a patient to read the eye chart quickly. What is now
known, however, is that the tear film quickly breaks up
after blinking, resulting in a substantial interblink degradation of vision. Japanese studies have documented that,
unlike normal eyes, within 3 to 4 seconds after a blink, the
VA in dry eye patients can decrease to 20/40 to 20/60,
leading to serious problems in reading and driving.24 These
experiences are difficult for the patient to describe, but
their effects on important activities of life can be appreciated more fully now. Continual attempts to compensate for
this phenomenon with rapid blinking lead to ocular
fatigue.
OF
OPHTHALMOLOGY
SEPTEMBER 2008
CURRENT CHALLENGES IN THE
DIAGNOSIS AND MANAGEMENT OF
DRY EYE DISEASE
IT IS A COMMONLY HELD OPINION THAT DED CAN BE
diagnosed largely on the basis of patient symptoms. Recently, a number of studies have called this impression into
question. Only a small percent of patients with DED have
been diagnosed.25 It has been documented that symptoms
of DED do not necessarily reflect the severity of the
disease.26 Clinicians have long know that many patients
without clinical evidence of DED, such as staining of the
ocular surface and decreased Schirmer test scores, are
highly symptomatic. Conversely, there is a subset of
patients with severe damage to the ocular surface with few
subjective symptoms. This lack of concordance between
signs and symptoms presents a problem not only in the
diagnosis of the disease, but also in assessment of severity
and in the design of clinical trials to evaluate the clinical
efficacy of drugs.22
Recent studies on corneal sensitivity may add some light
on these perplexing observations. Both animal and human
studies on the response to nerve damage to the cornea
have revealed that injured nerve endings respond by
developing microneuromas that may alter transducing
signals leading in dry eye states to an autonomous-like
discomfort.27 This may account for patient symptoms of
discomfort in the early stages of DED development in
which patients’ symptoms are out of proportion to observed tissue damage. Paradoxically, it has been observed
that inflammatory changes characteristic of a more severe
form of DED may result in decreased nerve sensitivity,
explaining the paucity of symptoms.6 This disconnection
between signs and symptoms is, as yet, not completely
understood, but must be factored into diagnostic criteria
and the design of clinical trials.22
Standard objective tests for DED also have shortcomings. The Schirmer test, which has been in widespread
clinical use for more than a century, has been criticized for
its variability and its tendency to exhibit wide intrasubject,
day-to-day, and visit-to-visit variation. As tear secretion
decreases in more advanced disease, the results become
more reproducible. In mild to moderate disease, however,
it has limited usefulness. Other standard tests in wide use
include the use of vital dyes to assess damage to the cornea
and conjunctiva. Those in general use are fluorescein for
the cornea and either rose Bengal or lissamine green for
the conjunctiva. Vital staining of the ocular surface,
although a measure of damage to the ocular surface is not
specific for DED, occurs in a substantial percentage of
normal subjects and is present in a minority of patients
with mild to moderate DED.22 In addition, reproducibility
in patients with DED and no change in treatment has been
reported as being relatively poor.28 This calls into question
its usefulness as a primary efficacy measure in clinical trials
for DED and suggests that its presence and degree may
VOL. 146, NO. 3
ADVANCES
IN
reflect short-term environmental influences as much or
more than underlying disease or effects of therapy.
Several classification schemas for DED have been developed.5 On a mechanistic basis, one distinguishing
between aqueous tear deficiency and evaporative dry eye
has been in use for more than a decade.3 Although this is
an important clinical tool, particularly in looking for
evidence of meibomian gland dysfunction of the lids, the
most common form of evaporative dry eye, increasingly it
is recognized that most cases of DED involve both types of
mechanisms. In creating a treatment plan, assessment of
severity of the disease is playing a more important role.5 In
the recent DEWS report, a severity scale has been introduced (Table 1). Based on the earlier Delphi panel article,8
it provides a useful clinical schema to aid in assessing
severity of disease; an accompanying set of guidelines for
treatment should prove useful for the clinician in making
practical decisions in the management of patients (Table 2).
Although treatment options have been limited largely
to over-the-counter tear substitutes and 1 Food and Drug
Administration (FDA)-approved therapeutic agent, cyclosporine A (Restasis; Allergan Inc, Irvine, California, USA)
several newer tear substitutes with therapeutic properties
have been marketed. These properties include stabilization
of the tear film, protection of the corneal and conjunctival
cells, reduction in evaporative tear loss by the introduction
of lipids, enhanced wound healing, and enhanced lubrication between lids and the ocular surface. In addition,
measures such as punctual plugs, environmental changes,
autologous serum for severe disease, and other emerging
strategies add to the spectrum of disease management
tools. There has been great interest in the use of omega 3
fatty acids either from diet or in the form of nutraceuticals
to treat DED. These compounds, which are present in fish
and green leafy vegetables, have anti-inflammatory properties. There are a few small, well-designed studies in
addition to anecdotal evidence to suggest their usefulness.29 Large-scale prospective clinical trials are being
developed to document their effects.
In addition to the use of cyclosporine (Restasis) to
modulate immune activity and to suppress inflammation in
DED, there is increasing evidence that the use of topical
corticosteroids as temporary or pulsed therapy can be useful
in reducing the damaging effect of inflammation.30 The
anti-inflammatory properties of doxycycline have been
demonstrated in animal models31; its benefits in the
treatment of meibomian gland dysfunction are well
known, and the anti-inflammatory effects of both systemic
and topical use are becoming increasingly recognized.
Reference to the treatment guidelines should aid in
making treatment selection. With the anticipated approval of more therapies directed to specific disease mechanisms in the coming years, the clinician will be called on
to make increasingly complex decisions to manage DED
effectively; it is probable that more than one therapeutic
DRY EYE DISEASE
353
TABLE 1. Dry Eye Disease Severity Grading Scheme
Dry Eye Severity Level
4a
1
2
3
Mild and/or episodic;
occurs under
environmental stress
None or episodic mild
fatigue
Moderate, episodic, or
chronic; stress or
no stress
Annoying and/or activitylimiting episodic
Severe and/or disabling
and constant
Conjunctival injection
Conjunctival staining
Corneal staining
(severity/location)
Corneal tear signs
None to mild
None to mild
None to mild
None to mild
Variable
Variable
Severe, frequent, or
constant without
stress
Annoying, chronic,
and/or constant
limiting activity
⫹/⫺
Moderate to marked
Marked central
None to mild
Mild debris, 2 meniscus
Lid/meibomian glands
MGD variably present
MGD variably present
Filamentary keratitis,
mucus clumping,
1 tear debris
Frequent
TBUT (seconds)
Schirmer score (mm/5 minutes)
Variable
Variable
Filamentary keratitis,
mucus clumping,
1 tear debris, ulceration
Trichiasis, keratinization,
symblepharon
Immediate
ⱕ2
Discomfort, severity, and
frequency
Visual symptoms
ⱕ 10
ⱕ 10
ⱕ5
ⱕ5
Constant and/or possibly
disabling
⫹/⫹⫹
Marked
Severe punctate erosions
MGD ⫽ meibomian gland disease; TBUT ⫽ tear film break-up time.
Must have signs and symptoms.
Reprinted with permission from Behrens A, Doyle JJ, Stern L, et al. Dysfunctional tear syndrome. A Delphi approach to treatment
recommendations. Cornea 2006;25:90 –97; and the 2007 Report of the International Dry Eye Workshop (DEWS), with permission from Ethis
Communications.
a
high indeed. In fact, the basis for approval of Restasis was
not on a primary efficacy endpoint but rather a secondary
one, that is, improvement in the Schirmer test results and
a correlated improvement in a symptom in a subset of
patients. As increasing evidence is appearing in the literature about the difficulties of using standard primary
endpoints such as vital dye staining, investigators have
been looking to other endpoints. This is a rapidly evolving
field in which design of clinical trials largely is proprietary
and, therefore, such information is not available for general scrutiny. A number of trends, however, are apparent
and are discussed in the recently published DEWS report.5
A principal problem encountered in all clinical trials is
the placebo effect of artificial tears on outcomes.22 This
refers to the observation that patients receiving a placebo
or drop with no active ingredients display notable improvements in most trials. The suggested reasons for this
include greater compliance in patients participating in
clinical trials, the lubrication effects of drops, and a
regression to the mean in subjects recruited on the basis of
findings that may be variable over time. The DEWS report
suggests that substituting a no treatment arm for a placebo
arm may be indicated.5
One innovative approach that attempts to harness the
short-term environmental effects on surface staining has
been the controlled adverse environment.32 In this experimental design, subjects preselected for prior clinical response, for example, staining in DED, are exposed to
adverse conditions such as wind and dry climate in a
agent will be required to provide optimal management of
patients.
CURRENT PROBLEMS AND FUTURE
PROSPECTS IN THE DEVELOPMENT
OF NEW THERAPIES
ADVANCES IN UNDERSTANDING THE MECHANISMS OPERA-
tive in forming and maintaining a normal tear film and the
pathologic breakdowns that occur in DED have led to a
variety of novel interventional strategies. These include:
secretogogues of aqueous tears, mucins and lipids, antievaporative compounds, immunomodulating agents that
have anti-inflammatory effects, corticosteroids, cellular
protective formulations, and tear film stabilizers. Although
most of the results of clinical trials are proprietary, published papers and abstracts presented at meetings suggest
that more than 20 products have undergone clinical
testing in the United States. As of this writing, only one
drug formulation has received FDA approval for marketing
as a therapeutic product for DED. It has been difficult for
sponsors to generate data that will meet the FDA’s criteria
of primary efficacy endpoints. These endpoints usually
include improvement in at least one sign and one symptom
and that these should be both statistically and clinically
significant. Given the information previously discussed
concerning the lack of concordance between signs and
symptoms in DED, the hurdle for obtaining approval is
354
AMERICAN JOURNAL
OF
OPHTHALMOLOGY
SEPTEMBER 2008
variation in presentation and variability over time. Another approach would be to identify target groups most
likely to respond to specific therapies, that is, those with
reduced but still measurable Schirmer test results in the
testing of a putative lacrimal secretagogue. The use of such
responder groups should increase the likelihood of demonstrating efficacy.
The DEWS report recommends that future trials using
surrogate markers for DED be considered.5 A surrogate
marker is a test that correlates with clinical evidence of
severity of disease. Tear osmolarity is one candidate discussed in the DEWS report. It is considered an established
marker and is considered “the central mechanism causing
ocular surface inflammation, damage and symptoms and
the initiation of compensatory events in dry eye.”5 Other
suggestions of the report for new efficacy endpoints include: an objective measure of functional VA, tear cytokines, more precise measures of tear stability, and altered
ocular staining schema allowing for minimal peripheral
corneal stain as seen in many normal subjects. Surrogate
markers will have to be validated to reflect disease severity
before they are suitable for clinical trials, but they represent a promising approach that circumvents the problems
of conventional endpoints such as ocular staining.
TABLE 2. Dry Eye Disease Treatment Guidelines Based
on Disease Severity
Severity Level
Description
1
Education and environmental/dietary
modifications; elimination of offending
systemic medications; artificial tear
substitutes, gels/ointments; eye lid therapy
If level 1 treatments are inadequate, add:
antiinflammatories, e.g., cyclosporine A,
topical corticosteroids; tetracyclines (for
meibomianitis, rosacea); punctal plugs;
secretogogues; moisture chamber spectacles
If level 2 treatments are inadequate, add: serum;
contact lenses; permanent punctal occlusion
If level 3 treatments are inadequate, add:
systemic antiinflammatory agents, e.g.,
cyclosporine A, prednisolone, methotrexate,
infliximab; surgery (lid surgery, tarsorrhaphy,
mucus membrane, salivary gland, amniotic
membrane transplantation)
2
3
4
Modified from the International Task Force Guidelines for
Dry Eye,6 and reprinted from the 2007 Report of the International Dry Eye Workshop (DEWS) with permission from Ethis
Communications.
specially designed temperature and humidity controlled
chamber while performing visual tasks requiring open eyes.
They are pretreated with either test drug or placebo and
are examined for response. This approach should be able to
determine pharmacologic effect in a short period. There is
a limited literature on this technology, and results of drug
trials generally are proprietary. As of this writing, no dry
eye drug approval using this approach has been announced. A possible limitation of the use of preselected
responders is the lack of generalizability to the entire DED
population. Nonetheless, this novel approach undoubtedly
will undergo further development and refinement.
Alternatively, others have tried to refine traditional
endpoints such as staining to specific areas, for example,
the central cornea, which has an effect on vision.33 In
addition, there are attempts to study variability in the
general population and that of DED patients to document
CONCLUSIONS
WHAT IS NOT KNOWN IS HOW EFFECTIVE THE NEW AGENTS
are that have undergone clinical trials; what is known is
that the methodology used to evaluate them is flawed. As
new information becomes available, designs for clinical
trials undoubtedly will undergo further evolution. This is
critical to surmount the regulatory barriers to successful
development of new, more efficacious treatment options
for patients with DED. As new products become available,
there will be a greater challenge to the clinician to
diagnose the disease more accurately and to establish
more effective treatment regimens for the different
stages of the disease. This augurs well for an improved
outlook for patients and greater professional satisfaction
for clinicians.
THE AUTHOR INDICATES NO FINANCIAL SUPPORT OR FINANCIAL CONFLICT OF INTEREST. THE AUTHOR IS AN OFFICER IN
Ocusense Inc, a company that has developed a tear osmometer. The author was involved in the design and conduct of study; data collection; analysis
and interpretation of data; and the preparation and review of the manuscript.
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OPHTHALMOLOGY
SEPTEMBER 2008
Biosketch
Dr Michael A. Lemp is a corneal specialist and the author of over 200 peer-reviewed scientific papers and editor of five
books. He received his undergraduate, medical and residency training at Georgetown University and fellowship at the
Massachusetts Eye & Ear Infirmary. Formerly a chair of ophthalmology at Georgetown, Dr Lemp current interests are in
tear/ocular surface disease, drug development, and clinical trial design. He is a Clinical Professor at Georgetown and
George Washington Universities.
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